1. Field of the Invention
The present invention relates to motor control systems, and more specifically, to a motor control system for centrifugals used in sugar refining machines and the like which implement graduated motor speed control to provide superior processing performance.
2. Description of the Related Art
Centrifugals used in refining sugar and similar substances are required to separate particulate sugar crystals from the syrup fraction of the massecuite, magma, or mother liquor that serves as the raw material in the refining process. When they are used in this way, they generally are classified as affination centrifugals. Also, they are used to spin moisture from the crystallized sugar during the cleaning process. When used in this way, they are called white sugar centrifugals.
The type of centrifugal used in these applications generally is the filter or basket centrifugal. FIG. 1 shows a typical centrifugal of this type. The centrifugal has a drive motor assembly 2 composed of a main drive motor 4 and a discharge drive motor 6 which alternately drive a perforated basket 8 disposed in a housing or curb 10. The basket 8 is usually about four feet high and three feet in diameter and can hold about 830 lb of raw sugar.
When used to separate impurities from the sugar product, the massecuite is loaded into the basket 8 and the main drive motor 4 spins the basket 8 to cause impurities in the massecuite to separate from the sugar due to centrifugal force. The screen of the basket 8 has apertures sufficiently small to retain sugar crystals included in the mixture, but the impurities pass through the perforations in the basket 8 and are drained off at the bottom of the curb 10. The sugar crystals accumulate on the screen and are held there by centrifugal force.
After the mixture has been separated, the basket 8 is spun down through a combination of regenerative motor braking and brake 12, and some sugar crystals adhere to the screen. These must be removed by a plow device 14 such as a knife or scraper that transits the screen surface of the basket 8 at a slow rotational speed when driven in reverse by the discharge motor 6. Once the sugar crystals have been removed from the sides of the basket 8 by the plow 14, a bell valve 16 in the basket 8 is raised or lowered (in the design shown in FIG. 1, it is lowered) to permit the sugar product to pass therethrough and be collected for further processing.
One stage of the additional processing involves the drying of a sugar slurry. At this point, it is necessary to remove moisture from the slurry by centrifugal spinning. This process involves a similar use of the centrifugal in which the slurry is introduced into the centrifugal basket 8 and excess moisture is spun out using the main drive motor 4. Then, the plow 14 is used to plow out the cake of dried sugar crystals formed during the spinning process while the basket 8 is driven by the discharge motor 6.
Prior art systems have used two-speed induction motors for main drive motor 4 to implement the above-described operations. Idealized speed and torque curves for a centrifugal cycle using such a motor are shown by traces 21 and 22 in FIG. 2, respectively, and trace 23 in FIG. 3 shows an actual cycle graph for a two-speed induction drive or current source modulated (CSM) centrifugal drive. The primary figure of merit in these systems is the cycle time, which is the amount of time required for the centrifugal to fully complete one sequential cycle of loading 26, accelerating 28, spinning 30, decelerating 32 and plowing out 34.
In the CSM drive arrangement, the main drive motor 4 is accelerated in its low speed mode until it reaches a desired loading speed (as shown in the Figure, about 250 rpm). The basket 8 is loaded, and the motor 4 is accelerated in high speed mode to spin the massecuite at about 1200 rpm to remove its impurities. Then, the centrifugal is decelerated, reversed, and plowed out as described above.
Operation of the CSM type drive has proven to be problematic for reasons of, for example, poor speed regulation capabilities, loss of kinetic energy due to high-slip operation and the brake 12, high maintenance requirements, and inability to use such motors in hazardous environments.
To partially overcome some of the above problems, prior art systems also have utilized pulse width modulated (PWM) drives for main drive 6, where the speed of the motor 6 is controlled by pulse width modulation. Trace 24 in FIG. 4 shows a typical cycle for a PWM centrifugal.
While these centrifugal systems function adequately, they have several disadvantages. For example, the amount of sugar that can be produced by a batch-type centrifugal such as those described above is necessarily limited by its cycle time.
Since the centrifugal runs in essentially a continuous operation, a cycle time decrease of a few seconds can result in a substantial increase in the efficiency (and consequently, productivity) of the machine. For example, a ten second decrease in a 150 second cycle time can result in an increase of 700,000 pounds of sugar per day over several centrifugals.
Within the cycle time, the loading and spinning time are generally constant for any centrifugal. While the accelerating, decelerating and plowing out phases are variable, they are limited at the lower end by the physical integrity of the centrifugal drive mechanism and by the quality of the finished product.